1. Field of the Invention
The present invention relates to electrical overvoltage surge protectors, used to protect telephone transmission lines against voltage surges, and more particularly, relates to spark gap protectors providing auxiliary or backup protection in case of failure of a main gas arrester device.
2. Description of the Relevant Art
Surge arresters or station protector devices generally include an insulated housing and contain a pair of spaced apart terminals with provision for maintaining a protector cartridge between the terminals. The protector cartridge may contain a pair of spaced apart carbon electrodes or a gas tube that defines an arc or a discharge gap therebetween in order to pass to ground excessive line voltages or currents in order to protect both the equipment connected to the line and the line itself. With repeated overvoltage conditions such as lightning strikes and transients, the carbon gap loses its effectiveness and the gas tube, although much more reliable, may also fail with continued use. Failure of a gas tube may be the result of the hermetic seals used to seal the gas within the tube becoming porous, thus allowing the gas to escape. This changes the breakover voltage of the protective gas tube arrester to a value which is greater than that desired for the line. Some of these protective devices include solder discs or fusing links which melt because of the excessive heat incurred during an overvoltage transient, causing the line terminal to short to the ground terminal. Many attempts have been made to make the backup or auxiliary protective spark gap survive multiple overvoltage transients without becoming completely shorted. This enables the line to be protected and in service, although tolerating a higher breakover voltage than is normally desired. Typical of a device which provides auxiliary air gap protection is disclosed in U.S. Pat. No. 4,158,869 issued to Gilberts on June 19, 1979. The device disclosed therein provides a pair of electrodes axially aligned with the gas tube and separated by an insulating spacer to provide an air gap therebetween. The air gap is maintained at a predetermined spacing such that that the breakdown voltage thereof is greater than the rated breakdown voltage across the arc gap of the gas tube, but less than the breakdown voltage across the arc gap should the gas tube fail open as by being vented to the atmosphere.
A similar device is disclosed in U.S. Pat. No. 4,132,915 which issued to Wilms on Jan. 2, 1979. The arrangement disclosed therein permits a cage finger to grasp a conducting ring spaced from a shorting contact member by a thin insulating spacer defining the auxiliary gap thereacross. Here again, the air gap is directly through the insulating material.
All of these devices operate in a similar manner and each device has a common shortcoming. Once the auxiliary arc gap has experienced an overvoltage, it provides a carbonized path which shorts out the two electrodes and therefore shorts the line it is protecting to ground, requiring a service technician to remove the protective device and replace it with a new one. It would be more advantageous to provide an auxiliary arc gap device such as for example a creepage path, which is capable of sustaining a plurality of overvoltage surges, thereby keeping the line open until a service technician is available to replace the defective arrester.
Therefore, it is an object of the present invention to provide a simple, inexpensive device for providing an auxiliary breakdown path in order to protect telephone or other communication lines from overvoltage surges when the gas tube arrester has failed in the open condition.
It is yet another object of the present invention to provide an auxiliary spark gap or a gas tube arrester that is capable of surviving a plurality of overvoltages surges.
It is yet another object of the present invention to provide a reliable backup creepage path device which is easy to install and is in parallel with a gas tube arrester and is readily replaceable therewith.
It is still a further object of the present invention to provide an auxiliary creepage path for use with a gas tube arrester which will not hamper other failsafe devices.